1. Field of the Invention
The present invention relates to optical waveguide devices comprising an optical waveguide substrate made of ferroelectric crystals.
2. Description of the Related Art
In the fields of optical measurement systems, optical communication systems, etc., it is known to manufacture optical waveguide devices by forming three dimensional optical waveguides in various electro-optics crystals.
As for the optical waveguide devices, optical branch devices, optical modulation devices, optical polarization devices, optical switches, multiplexers, and so on are known. The optical waveguide devices have such advantages as small in size, good stability, low necessary power and high speed signal transmission.
The inventors investigated the use of the optical waveguide device, which is manufactured by forming the three-dimensional optical waveguide of Y-branch type in the substrate made of LiNbO.sub.3, as a modulator for fiber optic gyroscopes. In this modulator, light impinging upon one end of the optical waveguide is divided by a ratio of 50:50% at a branch portion of the optical waveguide. One portion of the divided light is phase-modulated by applying a predetermined voltage. Phase differences due to Sagnac effects, which are generated by a rotation of the objected optical systems, are detected by the optical waveguide device. The thus obtained phase difference is introduced into a formula showing the relation between a rotation angular rate of the optical system and a phase difference to obtain a rotation angular rate.
The inventors further investigated an intensity modulator of Mach-Zehnder type formed by arranging an optical waveguide of Mach-Zehnder type in the substrate. In this intensity modulator, a light impinging upon the intensity modulator is portions of the divided. Respective divided lights are phase-modulated by applying a voltage thereto. Then, the portions of modulated light are interfered to vary the intensity.
During these investigations, the inventors faced problems mentioned below. In the modulator for fiber optic gyroscope, it is necessary to set the dividing ratio substantially equal to 50:50. However, especially in the modulator for fiber optic gyroscope, an admission range of the dividing ratio is very narrow. This is because, in the case that the dividing ratio is out of 50:50, the detected rotation angular rate is largely varied even if the rotation rate of the optical system is not varied.
For example, if it is assumed that the dividing ratio is temporarily varied from 50:50 to 40:60, the obtained rotation angular rate is varied about 20%. Therefore, in the modulator for fiber optic gyroscope, it is required to set the dividing ratio within 48:52.about.50:50. Moreover, the optical waveguide device is principally required to act stably along a wide temperature range. However, if an environmental temperature of the optical waveguide device was varied, the dividing ratio at the branch portion of the optical waveguide was temporarily varied largely. As a result, the dividing ratio of 50:50 was varied to 30:70, especially to substantially 100:0. Moreover, in addition to the light dividing ratio variation, a light insertion loss of the optical waveguide device occurred.
Further, in the Mach-Zehnder intensity modulator, if the light dividing ratio was varied, an extinction ratio is varied largely, and the light insertion loss of the overall optical waveguide device is temporarily increased at the same time.